The pneumatic valve positioner works on the force-balance principle (see the block diagram in the original). When the signal pressure entering the bellows increases, lever 2 rotates about its pivot, bringing the flapper closer to the nozzle. The nozzle back-pressure, amplified, feeds the diaphragm actuator chamber, moving the valve stem downward and turning the feedback lever (swing link) about its pivot. The feedback cam (eccentric cam) on the same shaft turns anticlockwise accordingly, rotating lever 1 via the roller and stretching the feedback spring. When the spring pull on lever 2 balances the signal pressure force on the bellows, the instrument is in equilibrium — a given signal pressure then corresponds to a given valve position.
This is the direct-acting mode. To reverse it, flip the cam from direction A to B. A direct-acting positioner increases output pressure as signal pressure rises; a reverse-acting positioner decreases it.
A direct-acting actuator with a reverse-acting positioner behaves as a reverse-acting assembly, and vice versa.
Editor's note: the original block diagram was not preserved in the web version.
